Nondestructive methods of evaluation have long been used to examine the interior portions of solid bodies for the detection of flaws or other defects. One such method in wide use today is that of ultrasonic testing.
In this method an ultrasonic transmitter is used to send a beam of sound energy beneath the surface of the solid body. Should this ultrasonic sound beam encounter any defects such as a crack, an inclusion of foreign material, a gap, etc., a reflected sound wave will be created which may be detected by a receiver also positioned above the surface of the solid body. Depending on the nature of the defect, the sound beam may be reflected in a direction which does not coincide with the current position of the receiver. For this reason, it is often necessary to perform multiple traverses of the surface of the body being inspected in order to insure that all internal flaws have been detected.
Moreover, even if a flaw should be detected by the currently used method of inspection, it is often not immediately possible to determine the depth of the flaw as the reflected sound wave may originate from a variety of depths beneath the surface of the solid body. This is especially cumbersome in the case of a pressure vessel having an internal cladding bonded to a substrate of other material. In this situation, the main zone of interest is the interface between the cladding and the substrate material, with flaws occurring deep in the substrate being of less consequence to the functionality of the pressure vessel. A prior art ultrasonic inspection system could detect flaws of widely varying depth in the first traverse, thus requiring additional inspections to determine if the flaw is an inconsequential defect in the substrate, or an unacceptable defect occurring between the cladding and the substrate.
One such situation in which very stringent inspection requirements are present is that of a nuclear reactor pressure vessel having a substrate of carbon steel and an inner cladding of stainless steel 3/8 inch (1.0 cm) in thickness. A flaw occurring between the stainless steel cladding and the carbon steel substrate could eventually result in a failure of the vessel and the release of the high pressure contents of the vessel. As such vessels are typically very large, and the inspection requirements very stringent, the need for multiple traverses in differing directions and with differing receiver/transmitter orientations results in an expensive and lengthy procedure.
What is required is a method and an apparatus for inspecting the zone of interest between the cladding and the substrate in a large pressure vessel which does not require multiple traverses to determine the existence of a flaw, and which is not susceptible to the detection of inconsequential flaws occurring in the substrate out of the zone of interest.